Over recent years, dramatic reduction of circuit dimensions to improve device performance and increase packing density in the integrated circuit (IC) industry has been achieved by the continuous innovation of optical imaging systems. Exposure tools with shorter exposure wavelengths and higher numeric apertures coupled with higher contrast resists have emerged successively to enable the replication of ever-shrinking features from reticles to resist coated wafers. For technologies with 150 nm features and smaller, it is rather difficult to provide manufacturable photolithography processes for 248 nm wavelength exposure tools without any optical enhancement techniques. Reduction of the exposure wavelength from 248 nm to 193 nm and to 157 nm demands enormous research and development efforts due to the limited material availability for projection lenses, reticles, and photoresists at these short wavelengths. Although significant progress has been made in 193 nm lithographic systems, some problems remain unsolved and currently prohibit these systems from becoming the technology of choice. These problems include the low etch resistance of 193 nm resists and pattern orientation dependent imaging caused by the presence of birefringence in the lens material used in 193 nm exposure tools.
As an alternative to developing shorter wavelength exposure systems, various complicated and costly optical enhancement techniques, such as phase shift mask and off-axis illumination, have been proposed to extend the lifetime of 248 nm lithography tools. The improvement in the resolution of 248 nm lithographic tools also may be obtained by using an inexpensive contrast enhancing layer (CEL) over the resist. This approach has been used in g-line, h-line and i-line photolithography to improve imaging resolution. Meldrum's diazo compound has been proposed as a photobleachable dye for deep ultraviolet (DUV) contrast enhancement materials (see Thompson L. F.; Willson G. C.; Bowden M. J.; Introduction to Microlithography, P. 252). However, it is known that Meldrum's diazo compound and its derivatives suffer from the problem of excessive loss during soft bake and, therefore, are not good candidates for use as DUV contrast enhancement materials (see Grant, B. D.; Clecak, N. J.; Twieg, R. J.; Willson, C. G.; IEEE Trans. Device Letters, Vol. ED-28 No. 11, 11/1981, P. 1300).